This invention relates to thermistors inclusive of chip-type thermistors (or "chip thermistors") of the kind which are commonly used in a temperature-compensating circuit or a temperature detecting element. More particularly, this invention relates to such thermistors having a pair of surface electrodes formed on one surface of a thermistor block so as to face each other.
Chip-type thermistors using semiconductor ceramics having a positive or negative temperature coefficient have been widely in use, and those with many different structures have been considered so as to be easily surface-mountable to a printed circuit board. The structure of some of prior art chip-type thermistors will be described first with reference to FIGS. 16-19.
FIG. 16 shows a prior art chip-type thermistor 71 with a pair of external electrodes 73a and 73b formed so as to cover both end surfaces 72a and 72b of a thermistor-forming base body (hereinafter referred to as the "thermistor block") 72 of a semiconductor ceramic material. The resistance of the thermistor 71 is determined not only by the resistance of the thermistor block 72 but also the contact areas of the external electrodes 73a and 73b with the thermistor block 72. The external electrodes 73a and 73b are usually formed by a dipping method, but this method tends to result in large variations in the lengths of the parts of the electrodes 73a and 73b covering the upper, lower and side surfaces of the thermistor block 72. Since there are variations also in the specific resistance of the thermistor blocks, the overall variations in the resistance of thermistors 71 thus produced are large, and it has been difficult to produce thermistors with a desired resistance value.
In view of the above, thermistors as shown in FIG. 17 at 75 came to be proposed, having glass layers 74 formed over the top, bottom and both side surfaces of a thermistor block 72. Since the external electrodes 73a and 73b contact the thermistor block 72 only through the end surfaces 72a and 72b of the latter, the resistance of the thermistor 75 is determined only by the resistance of the thermistor block 72 itself and the area of its end surfaces 72a and 72b. Thus, the variations in the resistance values of the thermistors can be reduced.
If the external electrodes 73a and 73b are formed by coating a conductive paste and subjecting it to a firing process, however, the materials of the glass layers 74 and the external electrodes 73a and 73b tend to diffuse into each other where they contact each other. If a portion of the glass layer 74 falls off as a result of such diffusion, as shown in the enlarged portion of FIG. 17 enclosed in a circle A, the outer electrode 73b may come to contact the thermistor block 72 directly. Since it is difficult to prevent such a diffusion phenomenon, there still remained the problem of obtaining thermistors with a desired resistance value.
Moreover, the variations in the resistance value of the thermistor blocks 72 have remained large, making it very difficult to obtain thermistors with highly accurate resistance values.
When thermistors 75 with different specified resistance values are to be produced, furthermore, thermistor blocks with different specific resistance values are required. It was thus even more difficult to accurately produce thermistors with different resistance values.
There have also been proposals to produce thermistors as shown at 77 in FIGS. 18A and 18B with inner electrodes 76a and 76b formed inside the thermistor block 72. In the example shown in FIGS. 18A and 18B, the two inner electrodes 76a and 76b are positioned in a face-to-face relationship with each other within a same plane at a specified height inside the thermistor block 72, one of the inner electrodes (76a) being connected to one of the external electrodes (73a) and the other inner electrode 76b being connected to the other external electrode 73b.
Such thermistors 77 are produced by a known kind of ceramic layering technology, the inner electrodes 76a and 76b being formed by applying a conductive paste on a ceramic green sheet by a screen printing method. Thus, the gap between the two inner electrodes 76a and 76b can be varied easily, even when thermistor blocks 72 of a same size are used, by adjusting the interval between the printed areas at the time of screen printing. In other words, thermistors 77 with different resistance values can be obtained fairly easily.
When the conductive paste is applied in a printing process as described above, however, the inwardly facing edges of the inner electrodes 76a and 76b are sometimes blurred and deformed, as shown enlarged in the elliptically marked portion indicated by arrow B of FIG. 18B. Since several ceramic green sheets are subjected to a firing process together after a conductive paste is printed thereon and they are piled up one top of another, furthermore, variations in the shrinkage accompanying the firing process also contribute to variations in the shapes of the inner electrodes 76a and 76b. As a result, it was also difficult to produce thermistors of this kind with inner electrodes accurately having a desired resistance value.
FIGS. 19A and 19B show a chip-type thermistor 78 disclosed in Japanese Patent Publication Tokkai 6-61011, intended to reduce variations in the resistance value, characterized as having a pair of rectangular surface electrodes 79a and 79b formed on the upper surface of a thermistor block 72 in a face-to-face relationship with respect to each other with a gap of a specified width in between. Outer electrodes 73a and 73b, as described above, are formed so as to cover portions of these surface electrodes 79a and 79b on the upper surface of the thermistor block 72. An insulating layer is formed additionally on the top surface of the thermistor block 72 so as to cover mutually facing edge portions of the surface electrodes 79a and 79b, as well as the portion of the top surface of the thermistor block 72 not covered by the surface electrodes 79a and 79b. These surface electrodes 79a and 79b can be formed accurately because the thin film technology can be used for this purpose. The requirement that the electrodes 79a and 79b must be rectangular is a drawback, however, when it is desired to produce chip-type thermistors with a very small resistance value because the separation between these surface electrodes 79a and 79b must be increased and this makes it necessary to use a larger thermistor block 72.